1. Field of the Invention
The present invention relates to an electroluminescence device, and a method for fabricating the electroluminescence device. More particularly, the present invention relates to a nanocrystal electroluminescence device comprising a polymer hole transport layer, a nanocrystal light-emitting layer and an organic electron transport layer wherein the nanocrystal light-emitting layer is independently and separately formed between the polymer hole transport layer and the organic electron transport layer, and a method for fabricating the nanocrystal electroluminescence device.
2. Description of the Related Art
A nanocrystal is defined as a material having a crystal structure at the nanometer-scale level, and consists of a few hundred to a few thousand atoms. Since the small-sized nanocrystal has a large surface area per unit volume, most of the atoms constituting the nanocrystal are present at the surface of the nanocrystal. Based on this structure, the nanocrystal exhibits quantum confinement effects, and shows electrical, magnetic, optical, chemical and mechanical properties different from those inherent to the constituent atoms of the nanocrystal. That is, the control over the physical size of the nanocrystal enables the control of various properties.
Vapor deposition processes, such as metal organic chemical deposition (MOCVD) and molecular beam epitaxy (MBE), have been conventionally used to prepare nanocrystals. On the other hand, a wet chemistry technique wherein a precursor material is added to an organic solvent to grow nanocrystals to a desired size has made remarkable progress in the past decade. According to the wet chemistry technique, as the crystals are grown, the organic solvent is naturally coordinated to the surface of the quantum dot crystals and acts as a dispersant. Accordingly, the organic solvent allows the crystals to grow to the nanometer-scale level. The wet chemistry technique has an advantage in that nanocrystals can be uniformly prepared in size and shape in a relatively simple manner at low cost, compared to conventional vapor deposition processes, e.g., MOCVD and MBE.
However, since nanocrystals prepared by the wet chemistry technique are commonly separated and are then dispersed in an organic solvent, techniques for forming a thin film of the nanocrystals in a solid state are required in order to apply the nanocrystals to electroluminescence devices.
In nanocrystal electroluminescence devices reported hitherto, the nanocrystals are used as luminescent materials, or have functions of light emission, in combination with charge transport. The first electroluminescence device employing nanocrystals was suggested in U.S. Pat. No. 5,537,000. The electroluminescence device is formed using one or more layers of nanocrystals as an electron transport layer, and preferably capable of emitting light. Accordingly, the luminescence wavelengths of the electroluminescence device are varied in response to the changes in the voltages applied to the device.
PCT publication WO/03/084292 teaches a device wherein a layer of an organic-inorganic hybrid matrix containing nanocrystals is disposed between two electrodes. Specifically, the device is fabricated by mixing nanocrystals and a low molecular weight hole transporting material, such as N,N-diphenyl-N,N-bis(3-methylphenyl)-(1,1-biphenyl)-4,4-diamine (TPD), with a solvent, and spin coating the mixture on an electrode. When the coating conditions and the mixing ratio between the nanocrystals and the hole transporting material are appropriately controlled, a nanocrystal layer is formed on top of a hole transport layer due to the difference in the intermolecular force or density between the nanocrystals and the hole transporting material. However, although the nanocrystal layer is formed on top of the hole transport layer, the hole transporting material is mixed with the nanocrystals in the transport layer. Accordingly, the overlying electron transport layer is in contact with the hole transport layer, and thus the hole and electron transport layers as well as the nanocrystal layer emit light. To solve this problem, the PCT publication discloses a technique for arranging a hole blocking layer on a thin film of the hole transport layer containing the nanocrystals, followed by forming the electron transport layer on the hole blocking layer. Meanwhile, the hole transporting material mixed with the nanocrystals has a low molecular weight. If a polymer is used as the hole transporting material, its solubility is low and thus the polymer is limited to material which can be dissolved in solvents which dissolve the nanocrystal. Although the polymer which can be dissolved are used, the solubility of the polymer is not sufficiently high, rendering it difficult to control the thickness of the nanocrystal layer and the hole transport layer.
U.S. Pat. No. 6,049,090 describes a device wherein a mixed layer of nanocrystals and a matrix as a light-emitting layer is disposed between two electrodes. According to the device, the matrix is selected to have a wider bandgap energy, a higher conduction band energy level and a lower valence band energy level than the nanocrystals so as to allow the nanocrystals to emit light well and trap electrons and holes in nanocrystals, thereby enhancing the luminescence efficiency of the device.
As stated above, the conventional electroluminescence devices employing nanocrystals as luminescent materials are devices wherein the nanocrystals have functions of light emission in combination with charge transport, are mixed with a hole transporting material to form a mixed layer, or are mixed with a hole transporting material and coated to form a nanocrystal layer separately formed on a hole transport layer due to the density difference depending on the processing conditions. However, since none of these conventional electroluminescence devices provide a pure nanocrystal luminescence spectrum, they have a problem of low color purity.